1. Field of the Invention
The present invention relates to a photodetecting circuit using an avalanche photodiode.
2. Related Background Art
An avalanche photodiode (APD) is a semiconductor photodetector which utilizes avalanche multiplication to achieve high sensitivity and response speed. Avalanche photodiodes are used as detectors for optical communications and low light level detections. Since such a device is comprised of semiconductor materials, the gain of an avalanche photodiode has a temperature dependence. In order to maintain a constant gain as a function of temperature it is necessary to adjust the bias voltage of the avalanche photodiode in a controlled manner. Various schemes have been proposed to effectively control the bias voltage of the APD.
One of those schemes is described in, e.g., Japanese Utility Model Laid-Open No. 61-181336 (FIG. 1). In this circuit, two APDs 16 and 11, having almost the same characteristics, are used. The APD 16 is shielded from light and is caused to break down by a current source (a high-voltage power supply 12 and a resistor 13). A breakdown voltage generated between a cathode and an anode of the APD 16 is divided by resistors 14 and 15 and applied to the other signal detection APD 11 as a bias voltage (reference numeral 7 denotes a load resistor). The breakdown voltage of the shielded APD changes depending on temperature. For this reason, this APD is used as a temperature sensor, thereby compensating for variations in the bias voltage of the signal detection APD. To obtain a predetermined multiplication factor, the breakdown voltage can be divided by a corresponding voltage division ratio to obtain a bias voltage, resulting in a simple arrangement.
Another scheme is described in Japanese Patent Laid-Open No. 55-068683. In this circuit, a breakdown voltage is measured during a set period of time. Within this period of time, the breakdown is detected by monitoring the current of flowthrough an APD. During a normal photometry, a bias voltage is set to be lower than the breakdown voltage, thereby maintaining a predetermined multiplication factor.